1. Field of the Invention
The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators horizontally. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. A reactant gas flow field is formed between the electrolyte electrode assembly and one of separators sandwiching the electrolyte electrode assembly for supplying a reactant gas along a surface of the electrode, and a reactant gas passage connected to an inlet or an outlet of the reactant gas flow field extends through the fuel cell.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is an ion exchange membrane. In the fuel cell, an anode and a cathode each including an electrode catalyst (electrode catalyst layer) and a porous carbon (gas diffusion layer) are provided on both surfaces of the solid polymer electrolyte membrane to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a unit of power generation cell. Generally, in use, the fuel cell has stack structure formed by stacking a predetermined number of power generation cells.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode, and a gas (reactant gas) chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating DC electrical energy.
In the fuel cell, generally, internal manifolds are provided for supplying the reactant gases, i.e., the fuel gas and the oxygen-containing gas to the anodes and the cathodes of the power generation cells in the form of stack structure. The internal manifold includes a reactant gas supply passage and a reactant gas discharge passage extending through the power generation cells in the stacking direction. The reactant gas supply passage and the reactant gas discharge passage are connected to an inlet and an outlet of a reactant gas flow field for supplying the reactant gas along the electrode surface.
In the structure, water produced in the power generation reaction tends to flow into the oxygen-containing gas passages (reactant gas passages) including the oxygen-containing gas supply passage and the oxygen-containing gas discharge passages for the oxygen-containing gas, and the water may be retained in the oxygen-containing gas passages. Further, water produced by water condensation or the like may be retained in the fuel gas passages (reactant gas passages) including the fuel gas supply passage and the fuel gas discharge passages for the fuel gas. Thus, the oxygen-containing gas passages or the fuel gas passages may be narrowed or clogged due to the retained water. Under the circumstances, the flows of the oxygen-containing gas and the fuel gas tend to be disturbed undesirably, and the power generation performance is degraded.
In this regard, for example, a method of operating a power generation apparatus using a solid polymer electrolyte fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 2001-332281 is known. In the conventional technique, the power generation apparatus using the solid polymer electrolyte fuel cell includes a fuel cell body and a humidifying apparatus. The fuel cell body includes an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode. Each of the anode and the cathode includes a catalyst layer and a porous gas diffusion layer. Power generation is performed in the fuel cell body by supplying the fuel gas containing hydrogen to the anode, and supplying the air as the oxygen-containing gas to the cathode. The humidifying apparatus is used for humidifying the air. For stopping operation of the power generation apparatus, firstly, humidification of the air is stopped, and then, the non-humidified air is supplied to the cathode for a predetermined period of time. Thereafter, operation of the fuel cell power generation apparatus is stopped.
Generally, the reactant gas flow field has a complicated flow field pattern such as a serpentine pattern such that gas grooves in the flow field become sufficiently long. At portions connecting the reactant gas flow field and the reactant gas passages, buffers are provided for smoothly supplying, or discharging the reactant gas.
In the structure, the reactant gas flow field is divided in the power generation surface. In the case of adopting the conventional operating method, the flow rate of the gas tends to be decreased in the buffers. Consequently, water may be retained in the reactant gas flow field in the areas just before the buffers. As a result, in particular, when the temperature is low, the retained water may be frozen, damaging the electrode catalysts of the electrolyte electrode assembly.